Evolutionary Consequences of Mass Extinctions

Abstract

Mass extinctions are relatively brief, global excursions of extinction rates above normal, background levels for a number of higher taxa. At least five major mass extinctions have perturbed the Earth’s biota over the past 600 million years, but current research has so completely emphasized causal mechanisms that the biological nature of the victims and survivors, and other evolutionary aspects of extinctions, have been neglected. Paleontological analyses suggest that selectivity during mass extinctions may be qualitatively different from patterns of extinction and survival during background times. This apparent indifference to background-regime adaptation means that mass extinctions can play a profoundly disruptive role in the evolutionary process. Taxa and morphologies can be lost not because they were poorly adapted by the standards of background processes (which constitute the bulk of evolutionary time), but because they occurred in lineages lacking the environmental tolerances or geographic distributions necessary to survive the mass extinction regime. For reasons that are still not clear, mass extinctions seem preferentially to remove taxa that are endemic, large-bodied, or tropical. Despite this disruption of background processes, some evolutionary trends persist across mass extinction boundaries; such long-term survival may require the chance occurrence within a single lineage of traits that enhance survivorship under both background and mass extinction regimes. By removing or reducing dominant groups, mass extinctions provide opportunities for diversification of taxa that had been minor constituents of the pre-extinction biota, channeling evolution in directions not predictable from situations established during background times.